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If you ran into this issue, the first thing you need to do is check if the PowerPivot solution is deployed to the web applications. You can do this by going to Central Administration > System Settings > Manage Farm Solutions and there you should see the solution powerpivotwebapplication.wsp. Make sure it is deployed to the correct web applications. If you are still running into issues, reinstall the rsPowerPivot addin on the frontend server where you are experiencing the issue. Uninstalling and reinstalling it makes sure all the dll’s are deployed correctly. Repairing the add-in in my case did not fix the issue.